This invention relates to a simplified inverter system for driving an a-c induction motor. More particularly, the invention relates to a highly efficient pulse width modulated inverter system for supplying single phase sinusoidal shaped voltage to a single phase a-c induction motor.
Speed control of single phase a-c induction motors has generally been accomplished by merely reducing the amplitude of the applied motor voltage. This decreases the motor's torque output and causes certain types of loads, such as fans, to operate at a slower speed. The penalty for this mode of operation is reduced motor efficiency due to "high slip" operation. Losses are often so high that special requirements are placed on rotor resistance and cooling methods.
Quasi-square wave (QSW) inverters, wherein the output alternating voltage is adjustable in both magnitude and frequency, are usually not used on single phase motors for two reasons. In the first place, since the inverter produces some form of square wave, a highly distorted current waveform, having substantial harmonic content, would result, causing excessive power losses in both the inverter and the motor, excessive motor heating and noisy (hum) motor operation. The second reason has been the high cost of providing such a controlled QSW inverter for a single phase motor.
The present invention alleviates both of these obstacles by providing a relatively simple, and inexpensive, pulse width: modulated (PWM) inverter system that generates genuinely sinusoidal single phase voltage and current waveforms while maintaining high efficiency in the inverter and motor.